One of the biggest advantages that digital cameras had when they were first introduced was the ability to change the sensor’s sensitivity to light (ISO) from shot to shot.
ISO is the sensitivity of the camera’s sensor to light. The lower the number (like 100) the less sensitive the sensor is to light and the higher the number (like 6400) the more sensitive it is to light.
The quality of the images at higher ISOs in the early digital cameras was not good. Not only was there a lot of grain (known as “noise”) but the images also lacked quality in colour and contrast.
One of the biggest steps forward in, not just digital photography, but photography in general, is the ability to use high ISOs to capture images in low light. Gone are the days of not being able to shoot 800 or higher speed film. Many digital cameras now are able to shoot three to four times faster (equivalent ISO of 6400 to 12800) with excellent quality. Some cameras have ISOs that can extend into the 3 million ISO range!
Each time you double your ISO (for example from 200 to 400), the shutter speed also doubles (for example 1/30 to 1/60). The following example explores different ISOs for one situation.
In this example at ISO 100, you would be able to achieve a hand-holdable shutter speed of 1/60th of a second at aperture F5.6 while you would be able to get the same shutter speed at F11 at ISO 400. This would allow you get more depth of field in the same situation.
Changing the ISO allows you to adapt to changing lighting conditions. Cameras have an Auto ISO setting where the camera will adjust the ISO when the shutter speed of your camera gets too slow. In fact, many cameras have Auto ISO as the default setting.
So why wouldn’t we just keep our cameras at high ISOs?
Although we are able to use different ISOs from image to image, there is a tradeoff. The higher the ISO, the lower the quality of your image. Higher ISO images have lower contrast and less vivid colours and, of course, more noise.
Not all digital cameras are created equal when it comes to the quality of high ISO images. Some are better than others so it is good to do a little research and comparison. It is also good to note that the larger the sensor, the better the quality at higher ISOs. So a 12 megapixel point-and-shoot camera with a sensor that measures 12mm by 18mm will generally have more noise than a 12 megapixel SLR camera with a sensor measuring 24mm by 36mm.
Generally, I try to keep my camera on the lowest ISO setting so I am getting the best quality image possible.
Workshop: Changing ISO
Start by putting your camera in aperture priority mode where you select the aperture and the camera selects the shutter speed. Find out how to change the ISO on your camera and adjust the ISO to 400. Make sure that Auto ISO isn’t on as it will change your ISO for you.
Put your camera down on a flat surface or a tripod and select an aperture such as f5.6. Take note of the corresponding shutter speed that the camera gives you to create a correct exposure.
Now change the camera’s ISO from 400 to 800. Your aperture hasn’t changed and your camera hasn’t moved but your shutter speed should be twice as fast as it was before. For example, if your shutter speed was 1/125 of a second, it will now be 1/250 of a second.
Without moving your camera, change your ISO back to 400 and your shutter speed should be what it originally was (in our example it was 1/125). Then change the ISO to 200 and the shutter speed should be half as fast (in our example 1/60)
Try taking a picture of the same thing with different ISOs and see what happens to your image when you use very high ISOs. You may have to look at the images on a computer screen to see the differences.
What you Need
A camera with lens that you can control the aperture and ISO
Remember
Use Aperture priority mode
Change the ISO to 400 and make sure that Auto ISO isn’t on.
Put your camera on a tripod or flat surface so it doesn’t move.
Set an aperture of f5.6 and take note of what the shutter speed is.
Without moving your camera, change your ISO to 800 and your shutter speed should be twice as fast.
Without moving your camera, change your ISO to 200 and your shutter speed should be half as fast as it originally was at 400 ISO.
Try the extremes of your camera’s ISO and download the pictures and look at the quality of the images.
